1. Field of the Invention
This invention relates to a method and apparatus for vapor phase synthesis of diamond. More particularly, it relates to a method and apparatus for producing a diaphragm of diamond which is used for a speaker or the like.
2. Description of the Related Art
Since diamond has an extremely high Young's modulus and a small specific gravity, its propagation speed of an acoustic wave is 18,000 m/s and this value is by far higher than those of other materials. Accordingly, diamond is an extremely excellent material for a diaphragm of a speaker, a headphone, a microphone, and so forth. Furthermore, a diamond film produced by vapor phase synthesis has a suitable internal loss irrespective of an extremely high propagation speed of the acoustic wave and in this sense, it is an ultimate material for a diaphragm.
Conventionally, the diamond diaphragm has been produced by the steps of depositing a diamond film on the surface of a substrate of Si, etc., shaped into the shape of a diaphragm (dome shape) by a hot filament assisted chemical vapor deposition (hereinafter referred to as HFCVD) and then removing the substrate by etching (for example, "Radio Technology", April, 1991, published by Radio Gijutsusha). FIG. 1 is a schematic view of a method of producing a diamond diaphram according to the conventional HFCVD. In drawing, reference numeral 1 denotes a diamond film; 2 is a Si substrate; 3 is a filament; 4 is a gas; and 5 is an exhaust system.
However, this method involves the following problems.
(1) The film deposition rate according to the HFCVD is as low as about 1 .mu.m/hr, and an extended period of time is necessary to form the film.
(2) The arrangement of the filament and a support structure are extremely complicated because a diamond film having a uniform thickness must be deposited on a curved plane.
(3) The filament must be frequently changed due to its elongation and deterioration (carbonization), and the film deposition conditions are not stable, so that film quality is not constant.
(4) A step of removing the substrate by etching is necessary after the deposition of the diamond film, and this etching, too, requires a long time.
(5) The same number of dome-like Si substrates as the number of the diaphragms to be produced must be prepared.
(6) Gas efficiency for the vapor phase synthesis of diamond is as low as only several percents.
For the reasons described above, the conventional diamond diaphragm is very expensive, and producibility is also very low.
It is therefore an object of the present invention to provide a method and apparatus for vapor phase synthesis of diamond capable of solving the problems described above and economically producing the diamond diaphragm with a high producibility.